The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging in open magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with other magnetic resonance imaging and spectroscopy systems, particularly those in which the B0 main magnetic field is orthogonal to the plane of the radio frequency coils.
Conventionally, magnetic resonance imaging procedures include disposing the patient in a substantially uniform, temporally constant main magnetic field B0. The resulting magnetization of the sample is manipulated with radio frequency magnetic fields that are applied to the examination region so as to manipulate the magnetization and produce magnetic resonance signals. These signals are received and used to produce images or spectra from the sample.
Often, the B0 field is generated along the central bore of an annular magnet assembly, i.e., the B0 field aligns with the central axis of the patient. In bore-type systems, ladder RF coils have recently been utilized for quadrature detection. The ladder coils have a mode that is perpendicular and a mode that is parallel to the surface of the coil. In a bore type magnet, both modes are readily oriented perpendicular to the B0 field. In this orientation, the coil receives resonance signals in quadrature.
However, not all magnetic resonance systems employ a horizontal B0 magnetic field. Vertical field or open magnetic resonance imaging systems typically include a pair of parallel disposed pole pieces which are often interconnected by a ferrous flux return path. Electrical coils for inducing the vertical main magnetic field are disposed along the flux return path or at the poles. Typically, the pole pieces are positioned horizontally such that a vertical field is created therebetween. Many advantages are realized with the use of vertical field systems, such as openness for patient comfort and greater patient accessibility for the physician.
The RF coils used to manipulate magnetization as well as receive the magnetic resonance signals are different for vertical field open systems than for bore type systems. In a vertical field system, the B0 orientation is directed across the patient as opposed to along the long axis of the patient, from head to toe. Useful RF magnetic fields, either linear or quadrature, are oriented perpendicular to B0. Thus, vertical field systems have used solenoid coils along the patient axis, a coil which is not useful for a bore magnet, unless the coil is appropriately oriented relative to B0. It is also desirable to have coils that conform to the openness requirements of the magnet. Volume coils for a bore type machine are often cylindrical, similar to the magnet bore. RF coils for a vertical field system are often parallel to the pole faces to maintain the desired openness of the magnet. Transmit RF coils for open systems usually consist of a pair of butterfly coils mounted parallel to the poles of the magnet. This conforms well to the open uses of the system.
Until recently, vertical field systems had a B0 field of 0.2 to 0.35 Tesla with proton resonance frequencies of roughly 8 to 15 MHz. As field strength is increased, the resonance frequency increases proportionally, requiring different coil design techniques. Also, at low fields, the receive coil thermal noise dominates while at magnetic fields of 1 Tesla or more, patient thermal noise dominates. Consequently, in high field systems, smaller receive coils or arrays of smaller coils are used to limit the patient volume contributing to the noise. Quadrature coils are usually used at higher fields because the patient noise seen by the orthogonal coils is also orthogonal resulting in a combined receive coil signal-to-noise improvement.
The present invention contemplates a new and improved RF coil for use in a vertical field system at higher field strengths which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus is provided. It includes an examination region through which a temporally constant B0 main magnetic field is generated. An RF transmitter transmits radio frequency pulses to a quadrature RF coil assembly to excite resonance in selected dipoles in the examination region such that the dipoles generate resonance signals at a characteristic resonance frequency. The quadrature RF coil assembly receives resonance signals from the resonating dipoles. At least one RF receiver demodulates the resonance signals from the quadrature RF coil assembly. The quadrature RF coil assembly includes a first RF ladder coil and a second RF ladder coil which are disposed in a parallel relationship to the pole pieces. The first and second RF ladder coils are rotated by 90xc2x0 relative to each other.
In accordance with a more limited aspect of the present invention, the RF coil assembly further includes a third RF ladder coil which is disposed partially overlapping the first RF ladder coil and is rotated by 90xc2x0 relative to the first RF ladder coil. A fourth RF ladder coil is disposed partially overlapping the second RF ladder coil and is rotated by 90xc2x0 relative to the second RF ladder coil.
In accordance with another aspect of the present invention, a crossed-ladder RF coil for an open magnetic resonance imaging system is provided. The crossed-ladder RF coil includes a first tuned ladder coil and a second tuned ladder coil which are disposed in a parallel relationship to each other and substantially orthogonal to the B0 field. The first and second ladder coils are rotated by 90xc2x0 relative to one another.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging in which a temporally constant main magnetic field is generated through an examination region and gradient magnetic fields are generated for spatial selection and position encoding. The method includes positioning a first ladder RF coil normal to the main magnetic field on one side of the examination region. Further, a second ladder RF coil is positioned parallel to the first ladder RF coil where the first and second ladder coils are rotated by 90xc2x0 relative to one another. Magnetic resonance is excited in dipoles of interest. Induced magnetic resonance signals are then received in quadrature with the first and second RF ladder coils. Finally, the received magnetic resonance signals are reconstructed into an image representation.
One advantage of the present invention is that it provides a planar RF transmitter which can be used at higher fields because it is a resonant structure.
Another advantage of the present invention is that a quadrature pair of ladder coils can be used on both sides of the examination region for better RF B1 magnetic field uniformity.
Another advantage of the present invention is that it allows for quadrature excitation and/or reception within the examination volume.
Another advantage of the present invention is that a pair of ladder coils used as receive surface coils can be sized for optimum coverage and signal-to-noise ratio.
Another advantage of the present invention is that the ladder coils can be arranged in arrays for good coverage of the examination region.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.